Folded slot antennae



J1me 1956 R; H. J. CARY 2,751,589

FOLDED SLOT ANTENNAE Filed June 9, 1952 3 Sheets-Sheet 3 m w r /77 411 g FIG. 6A 1/ A] g ga A; N 144. 2 40 3A 4b 35 4b 4 40 FIG. ll

1% In nfor I M Y y w United States Patent FOLDED SLOT ANTENNAE Rex Henry John Cary, Great Malvern, England, assignor to National Research Development Corporation, London, England, a British corporation Application June 9, 1952, Serial No. 292,529

Claims priority, application Great Britain June 20, 1951 38 Claims. (Cl. 343-767) This invention relates to folded-slot antennas and has particular reference to methods of reactanee compensation of such antennas in order to obtain a wide frequency band.

Antennas of the folded-slot type are well-known and have been described and used by H. G. Booker and others, particularly in connection with antennas for radar. The characteristics and advantages of such antennas, too, are well-known. One of these advantages is that of obtaining, by reactanee compensation, a wideband antenna for a given standing wave ratio. Various ways of obtaining such wide-band antennas by reactance compensation have been described previously but these appear to give a maximum bandwidth of :20% at the standing wave ratios acceptable for use in television and frequency modulation systems. For purposes where a larger standing wave ratio can be tolerated it is evident that some improvement in bandwidth will result from the actual relaxation of the standing wave ratio itself. This further improvement however is small in most cases and where bandwidths, for example, of the order of an octave at the larger standing wave ratio are required other methods must be adopted.

Additionally, in an aircraft, there is often required a suppressed antenna which can conveniently be provided by means of a slot in the aircraft skin backed by a cavity formed from part of the aircraft inside structure. Since the use of a cavity behind a slot effectively doubles its impedance the use of a folded slot has advantages in that folding a slot quarters this impedance. Moreover there is often insufficient room in the cavity behind the slot for a suitable probe with which to feed the slot and again the folded slot is advantageous since it can be directly connected to the feeder and then presents a reasonably suitable impedance to'the feeder of the type (e. g. coaxial) generally used.

Now, in most aircraft antenna installations a wideband is desired and, in a great many cases such as in some navigational aids, a larger standing wave ratio can be accepted than is normally required for television and frequency modulation systems. There is thus a requirement here for an improved wide band folded slot antenna, especially for use in cases of the larger standing wave ratio.

it is an object of the invention to provide a folded-slot antenna .with an improved bandwidth particularly when used with the larger standing wave ratios.

It is a further object of the invention to provide such a folded slot antenna which, if desired, may be connected to, and will match a low-impedance feeder such as the coaxial feeder.

A subsidiary object of the invention is the provision of a folded slot antenna suitable for installation as a suppressed antenna in the skin of an aircraft and having an impedance suitable for matching with and connecting to a coaxial type of feeder.

, -According-to the invention there is provided a reactancecompensated folded slot antenna in which the inherent parts in parallel.

reactanee of the folded slot antenna is compensated within desired limits over a frequency band by suitable slotfeeder reactanee in series therewith, characterised in that said series reactanee is made up of two parts, each part having a different reactanee-frequency characteristic such that the desired compensationis effected within the desired frequency band.

It is possible by suitable dimensioning and arrangement of the series slot feeder reactances of the folded-slot to obtain a resultant compensated reactanee-frequency characteristic which is a continuous curve oscillating about the zero reactanee ordinate and cutting it at least five times, the standing wave ratio remaining within predetermined limits over a wide frequency band.

According to a particular feature of the invention there is provided a reactanee-compensated folded-slot antenna asymmetrically fed. Such asymmetrical feeding is ob-- tained when, by physical offsetting of the feed point or electrical offsetting of the feed point of the folded slot, the slot feeder reactances are not symmetrical about the feed point.

After choosing a suitable feed point to give a desired slot impedance the frequency-band covered by the folded slot antenna may be determined by altering reactances of the slot feeders (e. g. by altering the lengths of the centre longitudinally along the middle of the slot, asymmetrical feeding consists of feeding between one side of the slot and the conductor by a feeder at a point less than quarter wavelength from one end of the slot. It will be appreciated that with such feeding the slot will comprise two One part consists of the slot feeder reactance looking in at the feed point towards one endof the slot, in series with the slot impedance as seen in that direction, and, the other the slot-feeder reactanee looking in at the feed point towards the other end of the slot in series with the slot impedance as seen in that direction. The differing frequency-reactanee characteristics of the two slot-feeders enable reactance compensation to be obtained over a wide waveband and, in particular, prevent simultaneous failure of feeding should both the slot-feeder reactances approach infinity together as they will do when the extremes of a wide frequency band are considered for a symmetrically fed antenna. The two different slot-feeder reactances do not approach infinity simultaneously and, since the parts of the folded slot in which they exist are in parallel, the feed impedance of the slot can be arranged never to approach infinity within the wide frequency band. Reactance compensation is thus possible over a wider range of frequency than hitherto. 1

In order to show the principles on which the invention is based and two methods by which it may be performed several embodiments will now be described as examples with reference to the accompanying drawings'in which:

Fig. 1 shows generally a folded slot,

Fig. 2 shows a general equivalent circuit of a folded slot,

Figs. 3 to 10 show typical arrangements of a reactanee compensated folded slot according to the invention,

Figs. 11 and 12 show the folded slot arrangement of Fig. 10 as used in a high speed aircraft, and

Fig. 13 shows a further folded slot arrangement'having an allround radiation pattern. v p v In the diagram of Figs. 1, 3 to 10 and 13 cross-hatching, going upwards from left to right, is conveniently used to prise serve as series reactances with which to compensate individually the corresponding partsof" the inherent reactance' of the folded slot antenna with" one of which each part' (slot-feeder) is inseries; The principleson: which su clrrea'ctancecompensation is based will now 'be further discussed with-reference to Figs: 1 and 2 of'the' drawings. Tn-Figi 1 a-folded slot 1 isshown fed at two points A, B. The folded slot 1 comprises a slot-WXY Z cut in a sheet- -of cond'uctiiig material and folded by the insertion ofv a centre-'- conductor ACDEF. The 'feed to the points AB may comprise anysuitable twin feeder and can be any- Where between the edge DE or CF 'ofthecentre conductor ACDEF and theedge WX, or ZY respectively of the slot WXYZ'.

A folded slot fed in this manner can be represented to.- a first approximation by an equivalent circuit as shown in Fig. 2. Looking in at the feedp'ointsA, B towards the. left 'ha-ndend of the slot, the slot feeder reactance is shown as X2, the radiation resistance as R1, and the infierent reactance as X1. XL and R-rand Xz-thev slot feeder,v

represent the part of the slot to theleft' of the feed points".-

A, B.- Similarly l'ookin-gin at the feed points'ABtowards the right hand end of the slot the slot feeder reactance is. s'hownas X3, theradiation resistance as R2, and the inherentreactance as X4.

Where a cavity is placed behind the slot WXYZ. the: cavitymay be'taken as introducing additionalreactances 'Xs-andXs; The connections to these reactances-are shown dottedbecause, as willbe" app arent as the description proceeds, the effect of the cavity may be. neglectedfrom the. point: of view of the invention and taken into account quite simply at alater convenient stage.

Now, taking the completely symmetrical case Where the centre conductor CDEF is symmetrical disposed in. the slotWXY-Z and the feed points A. and B; are inthe middle? of edges CF andZY respectively, it Willbe easily appreciated-that Xz=X-x; Rz=R1 and X4=X1. Thus whenever Xn reaches infinity there will'be no feedofenergy to the slotWXY-Zbecause the feederreactanceXa will also'reach. infinity. Therefore, the band of frequencies: over which the; folded slot will; act will be limited accordingly as X2- and'Xsisimultaneously approach: infinity. Reactancercompensation is. only possible on a very limitedbasis since: compensation" by making X3; the conjugate'ofXt-will also mearn the same thing as regards X2. andXrand failure will hesimultaneous owinggto the equality of X and X3.

If,.however,.either the" feed. points A, B,.or the centre conductor. CDEF, or both, arearranged asymmetrically so that the reactances X2 and X3 reach infinity atentirely diiierent. frequencies, feed of energy will. take place through either one slot feederor the other (i. e. to the left or right offeed points A, B) or at some frequencies, throughboth. In. any event the feed to the slot L will not fail over a wide band of frequencies. This asymmetry, willmean that Xr isalso different fromXt and therefore compensation either between X: and X4, or between X2 and Xi can be arranged, over a very wide band of frequencieswithout the feed tothe slot Lfailing. V I Referringagain to. the effect. of. adding a cavity behind the slotWXXZ, it will be seen. that if the dotted connections of Fig. 2 are assumed tobe made, then, suitable ad-- justments toslotfeeder reactances- X3 and Xzwill compensate X6 and X respectivelyin parallelwith X4 and X1, againrespectively.

. .i s-t l e e is s s i b e ssm d th In the following examples sake of simplicity that wherever a cavity is desired behind a folded slotthe necessary corrections in the reactance compensation will be made in accordance with the above and so no reference has been made to the use of cavities in the examples.

Reactance-compensation thus be achieved over a wide frequency band particularly where a variation of standing waveratio greater than acceptable for television and frequency transmissions modulation can be tolerated.

' Example I Fig. 3 shows a.half-wavelength folded slot comprising a slot 1, cutin'a; conducting sheet 2, in'which' a Hat strip 3 is symmetrically disposed in known arrangementa jacent feed points A and B are provided in the strip 3 and the perimeter of slot 1 respectively: The points A, B are at a distance I from centre line Cr It will be apparrent that there are two asymmetrical slot-feeders: One,

looking in at the points AB towards the left of the figure,

of physical and electrical length 11', and one, lookingin at;

the points AB towards the right of'the figure, of'physi'cal.

and electrical. length 12. The slot-feeder reactances seen willbe different. and will obviously vary differently in rel'ation to frequency; in particular each slot-feeder reactance will approach infinity at differing frequencies.

In the arrangement ofiFig. 1 it will be seen thatthe feed pointsA, B may. be chosen to provide a suitable impedance for use with different types of antenna feeder but:

that the position of complete symmetry where 1:0 and V the effective electric lengths. of [1 and [2 are equal must not be used; Since, however,. the shorter, physically, 16 81011 feeder [1 becomes, the poorer the feed of energy possibleinto the slot 1, from the shorter feeder [1, there is alimit' to the reduction in lengthof slot feeder [1, possible or desi'r able.

' Example 2 In: effecting. the required reactance' compensationit is 1: possible to-change the efiective electrical-lengthof the slot I slotrfeeder will be-l-ix/n. The effective electrical length. of?

feeder, lr of: the arrangement of Example l by inserting;

dielectric filling in the slot 1 to the left of the line drawn.

through the feed points A, B: If'the dielectricfilli'ng-has a dielectric. constant of rL- then theelectricallengthofthe either slot'feedermay be: changed in this'way, of course. Exampl'j' Referringv now to Fig. 4 it will be seen that-reactance;

compensation is: here being effected-by a methodin addi tion to those described-with referencetoEXample-labove,

by the adjustment of the dimensions of. the-centreeon.

ductor. In thelFig.v 4 the centre conductor to-the left of. the feed points. A, B may be made as. thin as mechanically desirable to obtain a required compensation. The slot to.

the left of the line through the'feed points A, B is filled withdielectric of dielectric constant n'to give anelectricah slot feeder length. to the left of the: feed. pointsA, B. of l1. /n. The width lstof. the centre conductor to thenright:

" of the feed points A, B is determined according to. the:

' compensating slotfeederreactance required.

Example 4 Example 5 Another method of adjusting the electrical lengths of the slot feeders. is by adding lumped or distributed reactance as indicated. in Fig. 6, or in Fig. 6A, where ae lumped" series rcactance ET is added in. series with the s lotvfeeder to the left of the feed points A, B. Conveniently, the series reactance lZImay comprise a suitable coil connected across a gap in the part of the centre conductor forming slot feeder length 11 of Fig. 6 or in the case of the connection shown dotted in Fig. 3 a condenser would be convenient.

Example 6 Example 7 Alternatively the slot feeder length 12 may be extended to join to the right hand'end of the slot 1 as shown in Fig. 8.

Example 8 In some cases it may be desirable to extend one of the slot feeders to a point beyond the end of the slot 1 to obtain a desired value for a slot feeder reactance. This may be achieved as shown in Fig. 9 where the slot feeder length 12 has been extended beyond the right hand end of the slot 1 and there connected to the surface 2 in which the slot 1 is cut.

Example 9 Yet another alternative arrangement is shown in Fig. 10 in which the slot feeder [1 has been made as a strip and the slot feeder 12 has been made as thin as mechanical- 1y allowable.

The actual electrical-mechanical interpretation as applied to a suppressed antennna for a high speed aircraft is shown in Figs. 11 and 12. The slot 1 is cut in the conducting surface 2 of the aircraft skin and covered on the inside with a sheet of dielectric material 4 which is fastened to the aircraft surface 2 by rivets 4a. The centre conductor 3is made up of a conducting strip 3A of length 11 and welded to a thin conducting rod 3B of length 12 thus forming the two slot feeders 3A and 3B of Fig. 10. The centreconductor 3 is fastened in the slot 1 by clamping it onto. the dielectric sheet 4 by clamps 4b. A coaxial feeder 6 is fastened and connected electrically by its sheath 6:: to the aircraft surface 2 at the feed point B and by its inner conductor 6b to the feed point A at the junction of the slot feeders 3A, 3B.

The arrangement of Figs. 10, 11 and 12 whilst giving two sectional fillings of dielectric of effectively different dielectric constant, i. e. plate of dielectric for the section 3A and plate of dielectric plus air for the section 3B, has

the advantages of simplicity and strength. It is customary in the case of slots used in aircraft to enclose the aircraft side of the slot by means of a resonant chamber. This is generally arranged to be a part of the internal bracing or structure of theaircraft.

Example 10 The invention is not limited to straight slots but may be used where it is necessary or preferable to have the slot 1 in other configurations. An example of this use is for an aircraft antenna in which an all-round radiation pattern is required. In such a case a folded slot 1 may be provided in the configuration shown in Fig. 13. The conducting surface 2 of the aircraft is shown by the crosshatching and the walls 5 of a resonant chamber which enclose the back of the aerial are shown in plan. In the structural design of the aircraft such walls 5 are conveniently made to form a useful part of the internal aircraft structure, which also provides the enclosing means, parallel to the aircraft surface 2, for the resonant chamber;

.The slot 1 is filled with a sheet of dielectric of suitable dielectric constant and on this sheet the strip 3 of the folded slot 1 is fastened. Terminals A, B are provided at the feed points and connect to a coaxial feeder. The strip 3' is displaced in the slot 1, in known manner, as shown; to provide a lower impedance for the slot 1 to match the particular feeder used without the necessity of moving the feed points A, B still further from the symmetrical position and so reducing the energy feed to the slot 1. Coupling may exist between the parts of the slot which extend in opposite ways from A, B fee'dpoints 'but this will merely modify the values of the inherent slot impedances (R1+X1) and (RH-X4) of Fig. 2 and will not affect the principle or successful performance of the invention.

It will be easily seen from the foregoing that a large number of variations of an asymmetrically fed folded slot antenna are possible without going outside the scope of the invention and that the variations illustrated and-described are by way of example only; the invention is not limited to such variations in carrying it into effect. Also the displacement of the strip 3 in the slot 1 and the configuration of slot 1 adopted in the example of Fig. '13

for the purposes stated are capable of application to any.

of the examples shown for similar purposes.

Further, it will be quite obvious to those to' whom this specification is addressed, how enclosing of one, side, of the slot by a resonant chamber will modify the reactance characteristics of the folded slot and what changes of compensating reactance are necessary to compensate for this; such modification will however in no way prevent the successful application of the invention in any particular case.

I claim:

1. In a reactance compensated folded slot'antenna in which the inherent reactance of the folded slot is compensated within desired limits over a frequency band by means of slot feeder reactance in series therewith, the combination of a conductive sheet having a slot therein, a centre conductor located in the slot, feeder connection means comprising a pair of feed elements connected one to a feed point on said centre conductor and the other to the edge of said slot adjacent the feed point on said centre conductor, the construction being such that the two slot feeders which result are connected in parallel to said feeder connection means and are of electrically dissimilar dimensions whereby within the operative fre-' quency band at least one of said slot feeders provides effective feed of energy to the slot from the feeder connection.

2. In a reactance compensated folded slot antenna in which the inherent reactance of the folded slot is compensated within desired limits over a frequency band by means of slot feeder reactance in series therewith, the combination of a conductive sheet having a slot therein, a centre conductor located in the slot, feeder connection means for connecting a source of radio frequency energy across the centre conductor and the adjacent edged the slot, the slot and the centre conductor thereby defining two slot-feeders in parallel connected'to the feeder con-' nection means, the construction being such that the two slot feeders which result are constructed with dissimilar electrical characteristics, the two slot-feeders Jthereby reaching at' separate frequencies within the frequency means of slot feeder reactance 'in series therewith, the

folded slot including a conductive sheet having a slot therein, a centre conductor located in the slot, feeder connection means for connecting a feeder across the centre conductor and the adjacent edge of the slot, the slot and the centre conductor thereby defining two slot feeders for feeding in parallel from the feeder connection means, and v dielectric filling means .for filling at least part of theaperture between the slot and the centre conductor, the

dielectric filling means being so chosen and located, the

feeder connection means being so located, and the centre conductor and slot configurations being so arranged thatthe two slot feeders extending each side of the feeder a r-515cc 7 connection means are. constructed to follow diflferentfrequency characteristics for the slot: feeder reactances to at separate frequencies within the frequency-band'- the value of reactance" which prevents the effective: feed ofienergyto the. slot from the: feeder connection.- meansvia thesiot feeder. Y

i 41' A reactance compensated folded slot antenna as claimed in claim 3 wherein the feeder connection means is located: nearer oneend portion of the slot than the oflien.

5.. A reactance compensated folded slott antenna as claimedin: claim 4,.wherein one end portion of the centre conductor is continuedand connects. to an adjacent endv portion ofthe slot.

6 A1 ireactance compensated folded slot antenna as claimed in claim 4, whereinoneendportion of the centre conductor is continued beyond: and over the adjacent end portion of the slot and connects at its extremity toth'econductive sheet.

7. A- reactance compensated folded slot antenna as claimed in claim 4', wherein the centre conductor comprises two parts, one extending along the. slot in one direction from the feeder connection means and one extending along=the slot in the opposite direction one part being smaller transversely than the other and the dielectric filling means being located in the aperture of the slot containing the one part of the centre conductor.

8 A reactance compensatedfolded slot antenna as claimedin. claim wherein thecentre conductor comprises--twoparts; one extending along the slobin one direction from: the feeder connection means and one. ex.- tending along the. slot in the opposite direction one: part being: smaller transversely than the other and the slot l'tltzcwvisecomprises two parts; one extending in one direc' tion from the: feeder connection means and one extending-*in: the opposite direction the part of the slot containing the smaller part of the centre conductor being smaller transversely than the part of the slot containing the other part of the centre conductor.

' 9;. A reactance compensated. folded slot antenna .as claimed in claim.- 3 comprising a lumped reactance. element connected in series with the centre conductor, the valueiofreactance: and location being'so arranged as to contribute to the electrical; characteristics of at least; one of the'slot feeders;

10.. A reactance' compensated folded slot: antenna as claimed. in claim 9 wherein the feeder connection means. is located nearer one end portionv of the slot. than the other; V

IL A reactance compensated. folded slot antenna asv claimed? in claim 9; wherein the lumped reactance element isconnected intermediately of the centre conductor in-tliat'part associated. with. one slot. feeder.

T2 A. reactance compensated folded: slot antenna as claimed in: claim 9; wherein the lumped reactance' element is connected between one end portion: of the. centre conductor: and one adjacent end" portion of the. slot.

1315A reactancecompensated folded: slot antennaasf claimed in claim 12, wherein. the lumpedi reactance elerrrent comprises acondenser. I.

14.- A reactance compensated folded slot antenna in which the inherent reactance'of the folded flsl ot is: com.- pensated within desired limits. over a frequency band by means of slot feeder reactance in series: therewith, thefolded slot including a conductive: sheet having a slot therein, a centre conductor located in the'slot," feeder connection' means for connecting a feeder across the-centre conductor and the adjacent edge of the'sl'ot, the slot and the centre conductor thereby defining two slot-feeders for feeding. in parallel from the feeder connection means, and a lumped reactance elementconncctcdii'i series with the'centre conductor; the valueof reactance and the location thereof relative to; the centre conductor; the feeder connection location; and the centre conductor and; slot configurations being so arranged that the" two slot feeders extending each side of. the feeder connection" means are. constructed to follow different frequency characteristics for the slot feeder reactances" to" reach at separate frequencies within the frequency band the high. value of reactance which prevents the effective feed of energy to the slot from the feeder connection means via the slot feeder.

1-5. A reactancecompensated folded slot antenna as claimed in claim 14 wherein the feeder connection means is located nearer one end portion of the slot'than the other.

16. A reactance compensated folded slot antenna as claimed in claim 14, wherein the lumped reactance element is connected intermediately' of the centre conductor. in that part associated with one slot feeder;

17. A reactance compensated folded slot antenna as claimed in claim 14, wherein the lumped 'reactance element is connected. between one end portion of the centre conductor and one adjacent end. portion of thecslot:

18'. A. reactance. compensated folded. slotv antenna as; claimed in claim 14., wherein the: lumped reactance el'ementcomprises.acondenser.

1 9. A reactance compensated folded slot. antenna in" which the inherent reactance of the folded slot is compensated within desired limits. over a frequency band by means of slot feeder reactance in series. therewith, the

folded slot including a conductive sheet having. a slot.

therein, a centre conductorlocated inthe slot, feedenconnection means for connecting a feeder across. the. centre conductor and the. adjacent edge of the. slot, the slot and the centre. conductor thereby defining! two slot-feeders for feedinginparallel from thefeeder connection means, the; feeder connection means being solocated and the centre conductor and slot configurations being so arranged that the two slot feeders extending from eachside. of thefeeder: connection means are constructed to be electrically dissimilar and therefore follow different frequency charac teristics for the slot feeder reactances to reach. at separate frequencies within'the frequency-band the. high value: of reactance which prevents the efiective feedof energy to the slot. from the feeder connectioncmeans'via the slot feeder.

20. A reactance compensated folded 'slot antenna as claimed in claim 19 wherein the feeder connection means is located nearer one end portion of the slot than the other.

21. A reactance compensated. folded slot antenna as: claimed in claim 2.01Wherein one: end'portion of the centre conductor is continued and connects to an ad jacentend portion of the slot.

22'. A reactance compensated folded slot. antenna. as claimed in claim 20, wherein one end portion of.v the centreconductor is continued beyond and-.- over. the adjacent end portion of the slot andconnects at its-..ex-,-

4 tremity to the conductive sheet.

23. A reactance compensated folded slot antenna as claimed; in; claim 20,: wherein the centre conductor'comprises; two parts, one-extending alongitheslotdn one .direc.- tion from theifeeder connection means andone' extending, along the slot in the oppositedirectiona onepart being: smaller transversely thanthe'other.

' 24 A reactancecompensated folded slot antmna. as: claimed in claim 20; wherein: an op'en-borcof conductive. material is positioned against the conductive-sheetso to enclose within its open edges that part of. the: sheet-in which the slot is located so forming a resonant cavity for theantenna.

25. A suppressed antenna-for use in aircraft: and fllfli; like having-conductiveexternali skins; comprisinga reactance compensated folded slot antenna in which: the

. inherent reactance of the folded} slot iscompensated withfeeder'connection means for connecting a -feeder across:

in desired limits over a frequency bandby suitable slotfeeder reactance in series therewith the folded slot i-neluding a-conductive sheet part of the aircraft-skim a slot th-er'ei-nha centre conductor located in the: slot;

the centre conductor and the adjacent edge of the slot and located nearer one end portion of the slot than the other, the slot and the centre conductor thereby defining two slot feeders for feeding in parallel from the feeder connection means, and a dielectric sheet closing the aperture of the slot, the centre conductor being located in the slot by being mounted on the sheet and comprising two parts to one part each side of the feeder connection means, one part being mounted in contact along its length with the dielectric sheet the other part having support members and being supported thereby out of contact with the dielectric sheet, whereby two electrically asymmetric slot feeders are defined extending each side of the feeder connection means and follow different frequency characteristics for the slot feeder reactances to reach at separate frequencies within the frequency band the high value of reactance which prevents the effective feed of energy to the slot from the feeder connection means via the slot feeder.

26. A suppressed antenna for use on aircraft and the like as claimed in claim 25, wherein an open box of conducu've material is positioned against the conductive aircraft skin so as to enclose within its open edges that part of the sheet in which the slot is located, so forming a resonant cavity for the antenna, said open box comprising parts of the aircraft structure behind the aircraft skin.

27. A suppressed antenna for use on aircraft and the like having conductive external skins comprising a reactance compensated folded slot antenna in which the inherent reactance of the folded slot is compensated within desired limits over a frequency band by suitable slot feeder reactance in series therewith, the folded slot including a conductive sheet part of the aircraft skin having a slot therein, a portion of the aircraft structure behind the skin comprising a box-like resonant cavity for the slot and having a fore-shortened partition member which forms two communicating compartments in the cavity, the slot being so located and shaped to be enclosed by both compartments, a centre conductor located in the slot, feeder connection means for connecting a feeder across the centre conductor and the adjacent edge of the slot and located nearer one end portion of the slot than the other, the slot and the centre conductor thereby defining two slot feeders for feeding in parallel from the feeder connection means, and a dielectric sheet closing the aperture of the slot, the centre conductor being located in the slot by being mounted on the sheet, whereby two electrically asymmetric slot feeders are defined extending each side of the feeder connection means and follow different frequency characteristics for the slot feeder reactances to reach at separate frequencies within the frequency band the high value of reactance which prevents the effective feed of energy to the slot from the feeder connection means via the slot feeder.

28. In a reactance compensated folded slot antenna in which the inherent reactance of the folded slot is compensated within desired limits over a frequency band by means of a slot feeder reactance in series therewith, the combination of a conductive sheet having a slot therein, a centre conductor located in the slot, feeder connection means comprising a pair of feed elements connected one to a feed point on said centre conductor and the other to the edge of said slot adjacent the feed point on said centre conductor, said feed points being located nearer one end portion of the slot than the other, thereby defining two electrically asymmetrical slot feeders connected in parallel to said feeder connection means, whereby within the operative frequency band at least one of said slot feeders provides effective feed of energy to the slot from the feeder connection.

29. In a reactance compensated folded slot antenna in which the inherent reactance of the folded slot is compensated within desired limits over a frequency band by means of slot feeder reactance in series therewith, the combination of a conductive sheet having a slot therein, a

centre conductor located in the slot, feeder connection means for connecting a source of radio frequency energy across the centre conductor and the adjacent edge of the slot, the feeder connection means being located nearer to one end portion of the slot than the other, the'slot and the centre conductor thereby defining two electrically asymmetrical slot feeders in parallel connected to the feeder connection means whereby the two slot feeders reach at separate frequencies within the frequency band the high value of reactance which prevents the effective feed of energy to the slot from the feeder connection means via the slot feeder.

30. A reactance compensated folded slot antenna in which the inherent reactance of the folded slot is compensated within desired limits over a frequency band by means of slot feeder reactance in series therewith, the folded slot including a conductive sheet having a slot therein, a centre conductor located in the slot, feeder connection means for connecting a feeder across the centre conductor and the adjacent edge of the slot, the feeder connection means being located nearer to one end portion of the slot than the'other, the slot and the centre conductor thereby defining two electrically asymmetrical slot feeders for feeding in parallel from the feeder connection means, and dielectric filling means for filling at least part of the aperture between the slot and the centre conductor, whereby the two slot feeders extending each side of the feeder connection means follow different frequency characteristics for the slot feeder reactances to reach at separate frequencies within the frequency band the high value of reactance which prevents the effective feed of energy to the slot from the feeder connection means via the slot feeder.

31. A reactance compensated folded slot antenna in which the inherent reactance of the folded slot is compensated within desired limits over a frequency band by means of slot feeder reactance in series therewith, the folded slot including a conductive sheet having a slot therein, a centre conductor located in the slot, feeder connection means for connecting a feeder across the centre conductor and the adjacent edge of the slot, the feeder connection means being located nearer to one end portion of the slot than the other, the slot and the centre conductor thereby defining two electrically asymmetrical slot feeders for feeding in parallel from the feeder connection means, whereby the two slot feeders extending each side of the feeder connection means follow ditfcrent frequency characteristics for the slot feeder reactance to reach at separate frequencies within the frequency band the high value of reactance which prevents the effective feed of energy to the slot from the feeder connection means via the slot feeder.

32. A reactance compensated folded slot antenna as claimed in claim 30, wherein the centre conductor comprises two parts, one extending along the slot in one direction from the feeder connection means and one extending along the slot in the opposite direction, one part being smaller transversely than the other and the dielectric filling means being located in the aperture of the slot containing the one part of the centre conductor.

33. A reactance compensated folded slot antenna as claimed in claim 1 wherein the centre conductor comprises two parts, one extending along the slot in one direction from the feeder connection means and one extending along the slot in the opposite direction, one part being smaller transversely than the other and the slot likewise comprises two parts, one extending in one direction from the feeder connection means and one extending in the opposite direction, the part of the slot containing the smaller part of the centre conductor being smaller transversely than the part of the slot containing the other part of the centre conductor.

34. A reactance compensated folded slot antenna as claimed in claim 31 wherein the centre conductor comprises two parts, one extending along the slot in one directiorpf rom the feederconnection means and one extending along the slot: in: theopposite direction; one part beingclaimed inclaim 29 wherein anopen box of conductive material is positioned against the'conductive'sheet so as to enclose fwithin its open edgesthat part of the sheet in which the slot is-located, so forming'a resonant cavity for the antenna.

3 7. A react'ancecompensated folded slot antennaas claimed 'in clainr 30' wherein an open box of conductive material is positioned against the conductive sheet so as to enclose within its open edges that part of the sheet in which theslot is located; so forming a resonant cavity for the antenna.

38-. A reactance compensated folded slot antenna asclaimed in claim- 31 wherein an open box of conductive material is positioned against the conductive sheet so as toenclose within i-ts open edgesthat fia-rt of the" sheet which the slot is located, so forming 31680113111? cavity for the antenna; 5 7

2,188,389 Cork Jan; 30,1940 2,431,124 Kees et a1. Nov. 18,1947 2,488,419 Lindenblad Nov. 15, 1949. 2,507,528 Kandoian- May- 16,1950 2,508,085 Alford May 16,, 1950' 2,555,443 Harvey June' 5, 1 951 FOREIGN PATENTS, I a

65, 5,045 Great Britain July 11, 1951' 664,150 Great Britain. Jan. 2,. 1952.

OTHER REFERENCES Slot Aerials, by D. A. Bell, Wireless World,'Eeb.

1948,pages58,57. r V j Iournal of Institution of Electrical Engineers, May 1946, vol. 93 Part III A, No, 4jSlot, Feeders and Sl'ot' Aerial'sfbyBailey', page s.6117, 618'. 

